Nervous System Overview

Nervous System Overview

• The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS includes nerves that branch out from the CNS to the rest of the body.

Central Nervous System (CNS)

• The brain receives and processes sensory information, initiates responses, and controls thoughts and emotions.
• The spinal cord conducts signals between the brain and the body, and controls reflex activities.

Peripheral Nervous System (PNS)

• The PNS has two divisions: the somatic nervous system and the autonomic nervous system.
• The somatic nervous system controls voluntary movements.
• The autonomic nervous system controls involuntary responses.
  • This includes the sympathetic ("fight or flight") and parasympathetic ("rest or digest") divisions.

Nervous Tissue

• Nervous tissue is made of nerve cells (neurons) and neuroglia.
• Neurons are the basic functional units of the nervous system.
• Neuroglia supports neurons.

Nerve Cells

• Neurons are specialized cells responsible for transmitting electrical signals throughout the body.
• A neuron includes the cell body, dendrites, and an axon.
• Dendrites receive signals.
• The cell body (soma) contains the nucleus.
• The axon transmits signals to other neurons or effectors.

Types of Neurons

• Multipolar neurons have one axon and many dendrites.
• Bipolar neurons have one axon and one dendrite.
• Unipolar neurons have one axon that branches into a peripheral process and a central process.
  • Sensory neurons are mostly unipolar.
  • Motor neurons are mostly multipolar.
  • Interneurons are mostly multipolar.

Ganglia

• Ganglia are clusters of neuron cell bodies located outside the CNS.
• There are sensory and autonomic ganglia.
  • Sensory ganglia are associated with cranial and spinal nerves.
  • Autonomic ganglia are associated with the autonomic nervous system and located in or near organs.

Synapse

• A synapse is the junction between two neurons.
• The synapse includes the presynaptic membrane and postsynaptic membrane separated by a cleft or space.
• Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.

Nerve Endings

• Nerve endings include receptors (receiving signals) or effectors (transmitting impulses).
  • Receptors receive sensory information and transmit impulses to the CNS.
  • Effectors transmit impulses from the CNS to skeletal muscle, smooth muscles, and glands. 

Receptors

• Receptors may be categorized by location, stimulus type, or histological structure.
  • Receptors for deep sensation are called proprioceptors.
  • Receptor for viscera are called visceroceptors.
  • Mechanoreceptors respond to touch, stretch, vibrations, and pressure.
  • Thermoreceptors respond to changes in temperature.
  • Nociceptors respond to pain.
•  Receptors for special senses include those for sight, hearing, taste, smell, and touch.

Nerve Endings (Effectors)

• Effectors transmit signals from the CNS to skeletal muscles, smooth muscles and glands, to produce a response
  • Motor end plates are located at the ends of axons and transmit impulses to skeletal muscles.
  • Autonomic motor endings transmit impulses to smooth muscles and glands.
  • Secretory endings cause glandular responses.

Neuroglial Cells (CNS)

• Neuroglial cells are supportive cells that aid in the function of neurons.
• There are several types in the CNS:
  • Astrocytes: exchange nutrients, support neurons, and provide structural support.
  • Oligodendrocytes: create the myelin sheath insulating axons.
  • Microglia: immune defense against invading microorganisms.
  • Ependymal cells: create, secrete, and circulate cerebrospinal fluid.

Neuroglial Cells (PNS)

• Neuroglial cells include Schwann cells and satellite cells.
  • Schwann cells form myelin sheaths around axons.
  • Satellite cells surround and support neuron cell bodies.

Autonomic Nervous System

• The autonomic nervous pathway includes two neurons, a preganglionic neuron and a postganglionic neuron.
  • Preganglionic neurons are located in the CNS (usually in the spinal cord).
  • Postganglionic neurons are located in autonomic ganglia outside the CNS.
• Preganglionic fibers of the parasympathetic system are longer and postganglionic fibers are shorter.
• Preganglionic fibers of the sympathetic system are shorter and postganglionic fibers are longer.
• The sympathetic and parasympathetic nervous systems have opposing effects on the body's functions.

Types of Ganglia

• Sensory ganglia: house cell bodies of sensory neurons.
• Autonomic ganglia: contain cell bodies of autonomic neurons, enabling autonomic functions.